Transconductance is often an important measure of performance parameters including, but not limited to, bandwidth, gain, and noise. Transconductance is an expression of the performance of certain electronic circuits, and traditionally refers to the ratio of output current to input voltage of a particular circuit, or mutual conductance. The term “transconductance” refers to herein as the control of an output current as a result of an input voltage.
In integrated circuits, it may be important for the transconductance, also generally referred to as Gm, of an electronic circuit to remain constant over one or more operating parameters as well as processing variations. The stability or robustness of transconductance of transistors may be an important design parameter, as it may be affected by many operation and processing conditions, such as temperature, carrier mobility, supply voltage, etc.
Transconductors, also generally referred to as Gm Cells, are typically important building blocks in any circuit design. Generally, transconductors are widely used in applications, such as Gm-C filters, Sigma-delta modulators, multipliers and so on. Also generally, in these applications, transconductors are key components that can limit a required dynamic range.
One conventional solution, i.e., a Gm Cell having the most wide linear operating range is shown in FIG. 1. The Gm Cell 100 shown in FIG. 1 includes an emitter degenerated input stage 110, a compression stage 120, a Caprio circuit 130, an emitter follower stage 140, and a current biasing circuit 150. It can be seen in FIG. 1, that the biasing current IB taken from a proportion to absolute temperature (PTAT) current reference to stabilize the Gm of the Bipolar Junction Transistors (BJTs). Further as shown in FIG. 1, VB is taken from source voltage reference that is capable of outputting stable current. Also as shown in FIG. 1, the supply voltage AVDD is connected to some internal regulated stable voltage source for improved power supply rejection (PSR) performance.
However, it can be seen that the Gm Cell 100 shown in FIG. 1, can require a supply voltage of nearly 3V in order to keep every transistor in a linear voltage. This is because VB is at least 3*Vbe+Vce, sat, which is around 2.4 V. For example, if loading circuitry needs 0.6V to stay in a high impedance mode, then the supply voltage required can be 3V, which does not include the extra room that is required for an output swing. Therefore, it can be seen that the Gm Cell 100 shown in FIG. 1 can require a supply voltage of at least 3V and this can be difficult to provide as the supply voltages shrink.